Boosting Oxygen Electrocatalytic Activity of Fe–N–C Catalysts by Phosphorus Incorporation

Nitrogen-doped graphitic carbon materials hosting single-atom iron (Fe–N–C) are major non-precious metal catalysts for the oxygen reduction reaction (ORR). The nitrogen-coordinated Fe sites are described as the first coordination sphere. As opposed to the good performance in ORR, that in the oxygen evolution reaction (OER) is extremely poor due to the sluggish O–O coupling process, thus hampering the practical applications of rechargeable zinc (Zn)–air batteries. Herein, we succeed in boosting the OER activity of Fe–N–C by additionally incorporating phosphorus atoms into the second coordination sphere, here denoted as P/Fe–N–C. The resulting material exhibits excellent OER activity in 0.1 M KOH with an overpotential as low as 304 mV at a current density of 10 mA cm–2. Even more importantly, they exhibit a remarkably small ORR/OER potential gap of 0.63 V. Theoretical calculations using first-principles density functional theory suggest that the phosphorus enhances the electrocatalytic activity by balancing the *OOH/*O adsorption at the FeN4 sites. When used as an air cathode in a rechargeable Zn–air battery, P/Fe–N–C delivers a charge–discharge performance with a high peak power density of 269 mW cm–2, highlighting its role as the state-of-the-art bifunctional oxygen electrocatalyst.

rotating rate of 1600 r.p.m. in O 2 saturated 0.1 M KOH solution. Specifically, the OER and ORR reaction pathways were determined by detecting the formation of HO 2 -, the ring electrode was kept at a constant potential of 1.5 V vs RHE in the OER or ORR potential region at a rotating speed of 1600 r.p.m. All potentials were transformed to RHE.
The TOF value of samples towards OER is calculated by equation (1): where j (A cm -2 ) is the current density, A (cm -2 ) is the geometric surface area of the electrode, F (s A mol -1 ) is the Faraday constant, and m (mol) is the number of moles of metal on the electrode.
Electrochemical impedance spectroscopy (EIS) measurements were recorded at the applied overpotential of 304 mV for OER with frequency from 0.01 Hz to 100 kHz at an alternating current voltage amplitude of 5 mV. ECSAs were estimated based on the C dl at non-faradaic potentials. By plotting the difference of current density (J) between the anodic and cathodic sweeps (J anodic -J cathodic ) at 1.35 V against the scan rate, a linear trend was observed. Accordingly, the C dl value was obtained according to the equation: Quantification of the active sites for ORR. The site densities of Fe active sites towards ORR in the Fe-N-C and P/Fe-N-C electrocatalysts were determined according to the method described by Kucernak et al. 1 The method is based on the adsorption and reduction of nitrite (NO 2 -) on the central Fe atoms. The site density and TOF were calculated using the following equations (2 and 3). where Qstrip (C g -1 ) is the excess coulometric charge associated with the stripping peak, the F is the Faraday constant (F = 96485 C mol -1 ), and n strip is the number of electrons associated with the reduction of one nitrite per site (n strip = 5), j k (mA cm -2 ) is the kinetic current density, = , Δjk = (j k (unposioned) -j k (posiopmed)), m cat , (g) is the mass of × catalyst. L cat is the catalyst loading during the reversible nitrite poisoning experiments (0.27 mg cm -2 ).
Zn-air battery. A home-built electrochemical cell was chosen to study the Zn-air battery performance of the P/Fe-N-C. The catalytic ink was loaded on carbon fiber paper (1 cm 2 ) with a loading density of 1 mg cm -2 . This carbon fiber paper and polished Zn foil were used as the air cathode and anode, respectively. A 6.0 M KOH aqueous solution containing 0.2 M Zn(OAc) 2 was employed as the electrolyte solution. All data were recorded from this cell on a Land CT2001A system at room temperature.
Computational details. Spin-polarized density functional theory (DFT) calculations were performed by using ab initio simulation package (VASP). [2][3][4] The generalized gradient approximation in the Perdew−Burke−Ernzerhof functional was adopted to describe the electron exchange and correlation energy, 5 and the frozen-core projector-augmented wave method with a cutoff energy of 500 eV was chosen to describe the interaction between core electrons and valence electrons. 6,7 The long-range vdw interactions between atoms is finely described by DFT-D3 correction method in Grimme's scheme. 8 the criteria of energy and force convergence are set to 1.0 × 10 -5 eV per atom and 0.02 eV Å -1 , respectively, for geometry optimization. And a Γ-centred Monkhorst-Pack k-point mesh grid of 3 × 3 × 1 was employed for all structural optimizations. 9 The vacuum space was 15 Å to avoid artificial interactions between periodic images in z direction. The OER process is divide into the four fundamental reactions as following: H 2 O + * = *OH + H + + e -(5) *OH = *O + H + + e -(6) *O + H 2 O = *OOH + H + + e -(7) *OOH = O 2 + H + + e - (8) where, OOH*, O* and OH * present the OOH, O and OH moieties on the adsorption site, respectively. The adsorption energy (△E ads ) of the key ORR intermediates, including *OOH, *O and *OH, was calculated relative to H 2 O and H 2 under conditions of T = 298.15 K, pH = 0, and U = 0 V (vs. SHE) according to following equations:  (14) For each element step of OER process, the Gibbs free energies were calculated using the following equation.
is the total energy of reactions obtained from DFT calculations. and ∆E ∆E ZPE ∆S represent the zero-point energy and entropic changes, respectively, which are obtained via vibrational frequencies computations with harmonic approximation and neglecting contributions from the slab. According to the computational hydrogen electrode (CHE) model proposed by Nørskov et al. 10 The free-energy change of 1/2H 2 → H + + e − reaction is treated to be zero at the potential of 0 and the free energy of proton and electron is set as the 1/2G (H2) . Because of the difficulties in the DFT calculations of open-shell triple O 2 , the free energy of gaseous O 2 (g) was calculated by G O2(g) = 2G H2O -G H2 + 4.92 eV. 11 Figure S1. Schematic of FeN 4 C 10 site, highlighting the first and second coordination spheres, where grey, blue, and golden balls represent the C, N, and Fe atoms, respectively.          Table S1. Elemental compositions of the Fe-N-C, P/Fe@N-C, and P/Fe-N-C, according to XPS measurements, contents of Fe in samples measured by ICP-OES.              Figure S21. Possible P-doping Fe-N-C (1-23) and pure Fe-N-C catalysts. Table S7. Fe-P bonds and formation energy of possible P-doping Fe-N-C and pure Fe-N-C catalysts. The gray-labelled structure is chosen to represent P-doping Fe-N-C through the standard of Fe-P 1 or P 2 < 3 Å and E f < -5 eV. The formation energy (ΔE f )was calculated as the following equation:

Catalyst C (at%) N (at%) O (at%) P (at%) Fe (at%) Zn (at%)
is he total energy of P/Fe-N-C system. , , , and are the chemical P/Fe -N -C C Fe N P potential of C, Fe, N, and P atoms defined as the total energy per atom in the stable elementary substance, such as graphene, Fe metal, N2 molecule, and elemental phosphorus. a, b, c, and d represent the numbers of C, Fe, N, and P atoms in P/Fe-N-C, respectively. Table S8. Fe-N bond lengths and corresponding strain in the P/Fe-N-C material.         Table S10. Fe-N bond lengths and corresponding strain in the P/Fe-N-C material.